Apparatus for dynamically balancing rotatable bodies



ug. 4, :1936. E, L, THEARLE 2,050,073

APPARATUS FOR DYNAMICALLY BALANCING ROTATABLE BODIES Original Filed Feb.l5, 19.33 l 2 Sheets-Sheet 1 f Y. f5

O 3 \V q n Inventor.

Evnesb Thea-fle, b9 #af y W4 is Abtor'heg.

Patented Aug. -4, 1936 UNITED STATES PATENT orrics APPARATUS FORDYNALIICALLY BALANC- ING ROTATABLE BODEE New York Original applicationFebruary l5, i933, Serial No.

656,878. Divided and this application September l, 1934,

4 Claims.

The present application is a division of my prior application forpatent, Serial No. 656,87t, led February 15, 1933.

The present invention relates to the dynamic balancing of rotatablebodies, and has for-.an object the provision of simple apparatus wherebythe amount ci unbalance of a rotating body and its location may bequickly and accurately determined.

A further and more specific object of the invention comprises theprovision in apparatus ci the above class or a circuit interrupting orcontrolling device which is capable of eiecting positive closure of alow potential circuit during acm curately determinable intervals oftime.

For a consideration of what l believe to be novel and my invention,attention is directed to the accompanying description and the claimsappended thereto.

In the attached drawings which are illustrative of my invention, Fig. lis a diagrammatic illus tration of the application of my improved appanratus to a machine ior balancing its rotor; lf'lg. 2 is a :front View cithe contact device or mechaH nisrn; llg. 3 is a vertical section of saidmechan f nism; Fig. e is a diagram; Fig. 5 is a longitudinal section ofone or the generators; Fig. i is an end view oi the generator; ifig. 7is a View in iront elevation oi one ci the springs used in the gen-lerator; Figs. il and 9 illustrate modified forms or the spring means ofFig. 6 and in different opere ative positions, and Figs. l to lll arediagrams or curves illustrating the theory of the balancing operations.

En connection with Fig. 1, l have illustrated in the interest or"simplicity an electric motor l,

having a rotor enclosed in a held frame as usual, and mounted on a shaft2 carried Yby bearing supporting pedestals 3. The machine frame andpedestals may have a common foundation. Mounted on two of the pedestals,any two will do ii there are more than two, are small electricgenerators i and Two generators are employed because it simplies thebalancing operations, but it is to he understood that a single generatormay be used and shifted from one pedestal to the other. This will reducethe cost of the apparatus initially, but such saving is largely orWholly offset by the cost of labor ne cessltated by shifting a generatorback and forth, properly aligning it, etc. t indicates a 'contactmechanism or device mounted on a suitable support, temporary orotherwise, at one end of the machine and is driven by a flexible shaft lfrom the main shaft 2 of said machine. The use of a Serial No. 742,411.7

(Gil. '33m-511) iiexible driving shaft l avoids the necessity ofaccurate axial alignment of the rotor of the motor and the contactdevice; also it results in a simple form of drive and one that canreadily be applied and removed. `The generators 4 and 5 are electricallyconnected to the contact device subject 'to the control of a manuallyactuated switch In circuit with one of the wires leading from thegenerators to the contact device is a sensitive, portable galvanometer twhich is calihrated in terms of movements of the pedestals dus tovibration thereof.

Pihe generator construction, which is more fully disclosed and claimedin my copending/ appllcatlcn, Serial No. 76,7255, filed April 28, 1936,and assigned to the General Electric Company, is illustrated in Flgs to9. In Fig. 5, l@ indicates a bracket which is rigidly bolted to abearing pedestal, and secured thereto, as by a screw thread, is a smallround rod or tube il. Mounted on the rod in spaced relation are twosprings G2 which are shovln in iront elevation in Fig. 7. These springssupport the permanent deld magnet i3 which is of crawler form andcomparatively massive and comprises an outer and an inner part with anannular air gap lil between the poles thus formed. .as it is desired tomaintain the field magnet in concentric relation to the anis of the rodil, the springs are arranged to be cuite rigid against any radialmovement of the field magnet but are quite elastic in -thel axialdirection. rJlhis is due to the iact that relative axial movements ofthe permanent mag net lleld and the armature are employed to gen-.-erate an electrical current, the magnet remaining practically stationerywhile the armature vibrates to and fro. .es shown, each spring whichacts after the fashion oi a diaphragm has four arms, and by means ofslits l5 are divided into two parts itl and itl. The part l@ is. securedtothe end head il of the magnet which is made of non-magnetic material.The part its is secured to the rod il. The right hand spring issimilarly constructed, one part being secured,to the non-magnetic endhead i8 and the other part to the rod il. By this arrangement, they heldmagnet and its armature to be described later can vibrate freely backand forth axially with respect to each other in response to vibrationsci the bearing pedestal but the field magnet and armature are restrainedagainst radial movements. In other Words, the rigid mounting of the neldmagnet results in a high natural frequency of' vibration in a radialdirection and a very low natural frequency oi' `mass such as the fieldmagnet will remain practically stationary'in space as the rod I I andthe armature carried thereby vibrate in an axial direction occasioned byvibrations of the pedestal.

It is evident that if the springs are to be quite stiff in a radialdirection, they must also have some stiffness axially or some resistanceto axial deflections. This results in a natural resonant frequencyofaxial vibration of the instrument which destroys or tends to destroyits usefulness at low balancing speeds. To avoid this diiliculty, meansare provided to completely or partially balance the axial stiffness ofthe springs, said means acting as if it were a negative spring. That isto say the means exerts a force proportional to the displacement .of thefield magnet and is in the same direction as the displacement.. Such ameans is shown in Figs. 5 and 6. Two blocks 20 have opposed V-shapednotches 2IIIl are secured by thin fiat elastic strips or springs 2| tothe field structure of the generator. Mounted on the rod or tube II is ablock 22 which has a pair of V-shaped notches 22* in its opposite faces.Situated between the blocks are stiff struts 23 having ground knifeedges at their ends which are seated in the V-shaped notches. The outerblocks 20 are pulled toward each other by adjustable tension springs 24of which two are provided. If the eld magnet is displaced to the rightrelative to the rod Il, for example, the struts 23 will tilt in such a.direction that they exert components of force on'the field structuretending to displace it still farther in the same direction. This actionis just the opposite of that of the ordinary spring, and by means of thef nuts at the ends, the tension of the springs 2d can be so adjusted asto just balance the positive stiffness of the field supporting springsI2. By this arrangement, the natural frequency of axial vibration vofthe generator may be made as low as desired.

In Figs. 8 and 9 is illustrated a modied form of the spring compensatingmeans in two of its positions. In this form, the struts 23 are elasticand are secured to thc central block or member 22 on the rod II. Theirouter ends may have knife edges as in Fig. 8 or be secured in the blocks20 as in Fig. 9.

The armature comprises an annular coil of wire 25, Fig. 5, mounted on anannular support 2lia of insulating material carried by a rigid spider'-25b supported by the rod Il. The coil is mounted in the air gap I4 andas the machine pedestal vibrates, the coil moves relatively to the iieldmagnet in an axial direction thereby inducing in the coil a lowpotential electro motive force which at any given instant isproportional to the velocity of vibration of the machine pedestal atsaid instant. Connected to opposite ends of the coil 25 are circuitwires 25c leading to the galvanometer 9, which elements, in combinationwith a cyclically operable current-interrupting device more fullydescribed in the following, comprise means for analyzing thecharacteristics of the current developed in the coil 25.

Referring now to Figs. 1 to 3, I have shown a current-interruptingdevice suitable for the purposes of the present invention. In theseiigures, 26 indicates a suitably supported stationary casing which has abearing for the stub shaft 26' driven by the flexible shaft 1. On theshaft is a cam 21 having a raised or cam surface 21* extending for halfof the circumference of the cam although a longer or shorter arc willnot interfere seriously with the operation of the de- "vice, A camexactly 180 long, as shown, gives the device maximum sensitivity alongwith other desirable characteristics and is therefor preferable. Apointer 28 is also' mounted on the stub shaft 26a for rotation with itand the body to be balanced. It is located diametrically opposite themid-portion of the raised portion of the cam. 29 indicates a ring orannular member mounted on a suitable bearing 30 on the frame 26 andcarries a xed contact 3! and a second contact 32 which' is actuated bythe cam.2-1 as it rotates. Since the ring member 29 is mounted to turnit is necessary to provide movable connections between the contacts andthe circuit wires. Suchconnections may be in the form of fiexible leadsor sliding contacts of ordinary construction. By reason of thisconstruction, the contacts 3i and 32 are in engagement and the circuitcontrolled thereby is positively closed during one-half of eachrevolution of the machine being balanced and of the cam. In the rear ofthe ring or member 29, Fig. 3, and fastened thereto is a spur gear 33which meshes with an idler gear 34, and the latter engages a pinion 35.'Ihe pinion is mounted on a shaft 3B which is turned by the small handle31, the purpose of this arrangement being to permit of shifting of thecontacts angularly about the cam. In other words, the midpoint of theclosed circuit period may' be made to come at any desired instant duringeach revolution of the rotor being balanced and of the cam. A pointer 38is fixed on the ring 29 for movement therewith and with the contacts 3iand4 32 and overlies a stationary graduated scale plate 39 carried bythe casing 26. By reason of this arrangement, the pointer 38 indicateson the scale 39 the angular position of the rotor being balanced and ofthe rotating pointer 28 the instant of the midpoint of the closedcircuit period A correct balance of any rotor having appreciable axiallength requires the addition (or removal) of two weights, one near eachend of the rotor. There are, therefore, four quantities to be determinedduring balancing, the amount of each weight and the angular position onthe rotor at which it should be applied.

In an article by T. C. Rathbone published in the Transactions of the A.S. M. E. for 1929, vol. 51, pages 267 to 284, there is described amethod whereby these quantities may be calculated after making thefollowing measurements or observations on each of the two chosenpedestals of the machine:

(1) Relative amplitude of vibration of the pedestal, the absoluteamplitude need not be known. This is indicated by the galvanometer whichis calibrated to indicate units of movement of the pedestal.

(2) Angular position of the rotor at the instant the pedestal is in theextreme right hand position in its vibratory motion, which is termed thephase angie". The manner in which these measurements may be obtained bythe use of ry novel apparatus will now be explained.

Curve D, Fig. l2 represents the horizontal, linear displacement of anypedestal due to vibration, plotted against the angular displacementrepresented by the peak 40 of curve D. Curve 75 V represents thevelocity oi' the pedestal due to vibration and is the ilrst derivativeor rate of change of curve D. The instantaneous E. M. F. induced by themoving coil 25, Fig. 5, of either generator is proportional to thisvelocity. The galvanometer t, Fig. 1, thus receives a series ofimpulses, each of a duration equivalent to 180 ci rotation oi the rotor,and one impulse per revolution thereby causing a. deiiection oi' itsindicating needle. the cam 2ln maintains the contacts 3I-32 closed forthis portion of each revolution. These impulses are represented by theshaded areas Il and 42 in Fig. 12. Whether these impulses are due togenerator 4 or 5 will depend upon whether the switch l, Fig. 1,completes the circuit o! one or the other of them. Ii' the contacts3I-I2 are now shifted angularly about the cam 21 by rotating the handle31 clockwise, Figs, 2 and 3, until the needle of the galvanometerregisters zero, the impulses will be as shown in Fig. 13 where that partof the impulse of the V .or E. M. F. curve below the horizontal line isequal to that part above the line.l This locates accurately the point 40as indicated by the pointer 38, Fig. 2, on the stationary circular scale39.

Moving the contacts 3| and 32 to eil'ect a zero reading of thegalvanometer also movedl the pointer 38 over the scale 39 and itsposition on said scale indicates the "phase angle.

Now if the contacts 3|'-32 are further shifted through 90 "from theposition last mentioned, the electric impulses received by thegalvanometer from a generator will be as represented by the shaded areasllt-M in Fig. 14, and the corresponding galvanometerl reading will beproportional to the amplitude of vibration. The galvanometer readingtherefore determines in suitable units the relative amplitude ofvibration of the pedestal being examined. The position oi` the switch 8will be determined by which ci the two pedestals is under consideration.

Although this method of analyzing imbalance vibrations may be easilyused, a complete understanding oi it requires familiarity with vectorsand their properties. Any simple sinusoidal vibration such as exists atthe pedestals of a rotating machine may be specified by means of asingle vector which is called the generating vector fortthat particularvibration. The readings oi' phase angle and relative amplitude" made onany one pedestal of a machine at any instant. completely specify thegenerating vector which describes and is characteristic of the vibratorymotion of the pedestal at that instant. This is illustrated in Fig. 10where, for instance, if any one pedestai gave a phase angle indicationoi' 45 degrees and an amplitude oi three units as indicated by thegalvanometer, the generating vector describing that vibration would beshown as the vector ii-l drawn at an angle oi' 45 and three units long.This vector is then a simple speeincation of the sinusoidal motionrepresented by the sine curve I-i--i, Fig. 11. Similarly, anotherpedestal or point on the machine, or even the same pedestal, afterapplying a balance weight, might indicate a phase angle (read by pointer@axon dial 39, Fig. 2) oi 135, Fig. 10, and an amplitude of two units.This motion would be specified by vector 6 2 which describes the-sinecurve 2-2-2 of Figjll.. It the vectors (i-i and l-Z are thought oi asrotating, at rotor speed about point 0, the actual displacement of thevibrations they repre sent would be equalv to the projections oi' theThis is due to the fact that.

3 vectors on the vertical line 3 0-3. In dealing ,with these vectorquantities, each one hasy two dimension`s,an angle (phase angle) and amagnitude or amplitude.

Asbefore indicated the purpose of the apparatus here described is todetermine the relative amplitudes of vibrations and what will be calledthe "phase angles of these vibrations o1' a plurality of points on themachine being tested and under va-rious conditions of unbalance.

The position of the rotor, during its rotation,

at the instant any chosen point on the machine is in some previouslydened momentary state during its vibration will be .called the phaseangle of the vibration of said point under the existing conditions.

'I'he chosen points are the points of attachment of the generators whichmay be setup to operate horizontally, vertically or in any desiredposition.

The previously defined momentary state during the vibration of a chosenpoint might be for example (a) When the moving coil 25, Fig. 5, is inits innermost position relative to the field, or (b) In its outermostposition, or

(c) In its mid position, moving outward, or,

(d) I n its mid position, moving inward.

When using the apparatus here described, the most accurate readings maybe made if this momentary reference state in the vibration is taken asone of the extreme or end'pcsitions of the moving coil 25. For the sakeof simplifying the description, I will arbitrarily define the momentaryreference state to be when the moving coil When the center of thecontact period occursat this instant, the galvanorneter 9 will indicatezero, and it Willbe assumed that the electrical connections are so madeas to polarity that, starting with this setting of the contactor,rotation of the gear wheel by handle 3l, Fig. 2, in a right handdirection will cause the meter reading to increase (toward the right)from zero. Then itwill be imown that if the meter reads zero and righthand rotation oi the gear wheel 35 decreases the meter reading, themoving coil is at its innermost position, and the correct reading mustbe obtained by rotating the ring 29 and contacts 3i, a2, 180 degrees, bymeans of the gear wheel 35 and handle 31. When this is done, the meterreading will again be zero, and right hand rotation of handle 31 willincrease said reading. The operator will then know that the midpoint oithe contact period is occurring at the instant the moving coil is in itsextreme outward position, which I have arbitrarily assumed to be themomentary reference state during the vibration of the point in question.

The phase angle" of the vibration of the point in question may bemeasured from any arbitrarily chosen zero, and will then be given by thereading of pointer, 38 on scale 39.

The pointer 38 with contacts 3i and 32 is then shifted 90 by turninghandle 31 to thevright. The meter 9 will then indicate a maximumreading, which reading is taken as the relative amplitude of vibrationof the point in question.-

The apparatus thus makes it possible to determine, for any chosen pointon the machine and under any condition o! vibration, the two quantitlesf (a) Phase angle, and (b) relative amplitude, which denne thecharacteristics oi' the vibration of said point.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. In apparatus for determining the unbalance of a rotating body, asource of low potential current having characteristics determined bysaid imbalance, means dependent on the cyclical interruption andrestoration of said current at accurately determinable intervals foranalyzing the characteristics oi the current and a current-interruptingdevice in series with said source and said means comprising a rotatablesupport, a movable and a stationary contact mounted on said support,means' for rotating said support, a scale means, one part of which ismovable with said support to indicate the angular position oi' saidcontacts, and a cam rotated in synchronism with said body, said cambeing eiective to produce positive closure of said contacts during aportion of the rotation of the cam determined by the position of thecontacts.

2. In apparatus for determining the unbalance of a rotating body, asource of low potential current having characteristics determined by thesaid unbalance, means dependent on the cyclical interruption andrestoration oi said current at accurately determinable intervals foranalyzing the characteristics of the current and a currentinterruptingdevice in circuit with said source and said means comprising a camrotated in synchronism with the body, a rotatable support support aboutits axis, a pointer attached to tho support and a stationary graduateddial cooperating with said pointer to indicate the angular position oisaid contacts, said cam being positioned to produce positive closure ofsaid contacts 5 during a portion of the rotation ot the cam determinedby the position of the contacts.

3. In apparatus for determining the unbalance of a rotating body, anintermittently operable circuit-interrupting device comprising a cam l0adapted to be rotated in synchronism with the body, a pair oi contacts,one oi which is actuated by the cam to effect positive contact with theother during a. predetermined portion of each revolution ot the cam,means for moving the 15 contacts anguiarly with respect to the camthereby to determine the period o! closure of said contacts, and a scalemeans, one part oi which is movable with said contacts to indicate theangular position thereof.

4. In apparatus for determining the imbalance of a rotating body, anintermittently operable circuit-interrupting device comprising a camadapted to be rotated in synchronism with the body, a rotatable supporthaving substantially the same axis of rotation as said cam, a pair otcontacts mounted on said support, one of said contacts being actuated bythe cam to eiiect positive contact with the other during a predeterminedangular rotation of the cam, means for turning the support on its axisto change the relative positions of the cam and the contact actuatedthereby, a stationary scale, and a pointer `mounted. on the support forrotation therewith to indicate on the scale the angular position oi! thecontacts.

ERNESTL msnm.

